Technical Abstract:
Introduction: Developing antimicrobial resistance to new and older drugs is of global concern. However, the growing trend of multiple resistance, particularly multiple resistance involving both new and older drugs, will exacerbate the problem. The National Antimicrobial Resistance Monitoring System - Enteric Bacteria (NARMS-EB) was established to monitor emerging resistance in food borne and commensal bacteria of human and animal origin. Methods: As part of the animal arm of NARMS, Salmonella isolates were submitted from raw product collected at slaughter and processing plants (designated SI) or from feces or tissue samples collected from diagnostic submissions (designated DI) from chickens (CH), cattle (CL), swine (SW) and turkeys (TK) for 1997 through 2000. Antimicrobial susceptibility testing was conducted using the Sensititre**TM System (Trek Diagnostics, Inc.) as per manufacturer's directions. Antimicrobials were selected and configured in a 96 well custom made panel. Results were analyzed by year, type (SI or DI), source (CH, CL, SW, TK), and serotype. Total percent pan-susceptible and resistance to only 1 antimicrobial were combined for this analysis. Multiple resistance was defined as resistance to > 2 antimicrobials Results: Overall, SI from CL, SW, and TK were more susceptible than DI regardless of year. For CH isolates, however, multiple resistance was 34% in 1997 and 1998 and 33% in 1999 and 2000 among SI while multiple resistance among DI was 23%, 40%, 26%, and 26% for 1997-2000, respectively. Regardless of type, turkey isolates had more multiple resistance than CH, CL, or SW. The least multiple resistance was observed among CL samples collected from slaughter. The most common resistance pattern among all sources was to Streptomycin, Sulfamethoxazole and Tetracycline. Resistance to more antimicrobials was observed among DI. Analysis within source by year was variable and no trend among sources was detected. The top 5 most frequent serotypes also varied widely between sources by year. Conclusions: These data indicate that multiple resistance is less in CL, SW and TK SI when compared to DI. This is not unexpected as submissions to a diagnostic laboratory are often made after an animal is ill or dead, and depending upon source, may have been treated for prior illness. The reverse observed for CH isolates was not expected. However, it is possible that this reflects submission of samples from earlier periods in production prior to extended exposure or use of antimicrobials or variations in serotype. The most common multiple resistance pattern among all isolates is notably to older drugs. However, when multiple resistance included newer drugs, resistance was also observed to the older drugs. Molecular analysis of multiple resistant cassettes will assist in further analysis of the data.